The present invention relates to a technology effectively applied to a high frequency power amplification circuit which is used in a radio communication system, such as cellular phone, and amplifies and outputs high frequency signals and an electronic component and a radio communication system which incorporate the power amplification circuit. More particularly, the present invention relates to a technology for enhancing the stability of control loops and the response to change in request-to-send level in a radio communication system having a detection circuit which performs the detection of output power, required for feedback control of a high frequency power amplification circuit, by current detection.
In general, the transmission-side output portion of radio communication equipment (mobile communication equipment), such as cellular phone, incorporates a high frequency power amplification circuit which amplifies modulated signals. Conventional radio communication equipment is provided with an automatic power control circuit (APC circuit). (Refer to Patent Document 1, for example.) This is for controlling the amplification factor of the high frequency power amplification circuit so as to obtain output power corresponding to the level of a request-to-send from a baseband circuit or a control circuit, such as microprocessor. The APC circuit detects the output level of the high frequency power amplification circuit, and compares the detection signal with a request-to-send level (output level instruction signal). Then, the APC circuit generates an an output control signal Vapc for feedback-controlling the high frequency power amplification circuit. In general, detection of output level is carried out using a coupler, a detector circuit, or the like. The detector circuit is usually constituted as a semiconductor integrated circuit separated from the high frequency power amplification circuit.
The coupler is an element which detects the output level through capacitance produced between an output line (microstrip line) formed on a discrete component or an insulating substrate (module substrate) and an electric conductor placed in parallel with it. The coupler is larger in size than elements formed on a semiconductor chip. The details of coupler (directional coupler) is found in, for example, “Foundations and Applications of Microwave circuit” (Sogo-Denshi Publisher, 1997, pp. 191–193).
Output level detection methods for conventional high frequency power amplification circuits use a large number of semiconductor integrated circuits or electronic components separated from the high frequency power amplification circuit. This makes reduction in module size difficult. If a coupler is used, reference voltage is sometimes applied to an end of the coupler for the enhancement of detection sensitivity. In this case, reference voltage must be optimally set and voltage and the like must be adjusted in accordance with variation from component to component. This increases a burden on set makers and is a problem. Use of a coupler also poses another problem: a relatively large power loss is produced.
Consequently, the applicants made an invention related to a radio communication system of current detection type and made an application for it (Japanese Patent Application No. 2000-523757). The radio communication system is provided with transistors for output detection which receive the input signals of transistors for power amplifier for amplifying high frequency signals and pass a current in proportion to the current passed through the transistors for power amplifier. The radio communication system is also provided with current mirror circuits which duplicate the currents in the transistors for output detection. The current duplicated by the current mirror circuits is converted into voltage to obtain a detection signal for output level. The detected output level is compared with the level of a request-to-send, and thus the output level is controlled.
[Patent Document 1]
Japanese Patent Prepublication No. 2000-151310